1) Field of the Invention
The present invention relates generally to a flat panel X-ray detector, and particularly to a flat panel X-ray detector having improved attenuation accuracy and dynamic range resulting from reduced internal scattering.
2) Background of the Problem and Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
One of the important novel aspects of this invention is our understanding of the cause of a long-standing problem and how the problem can be corrected. Until now, the problem of scattering due to the detector components and the significance of said scattering have only been partially understood, and therefore steps have not been taken to rectify the problem. The result is that inaccurate measurements of X-ray attenuation are unknowingly accepted. If one is concerned only with (qualitative) images, this may be acceptable. However where quantitative information is needed (such as in computed tomography), inaccurate measurements of X-ray attenuation are unacceptable.
In Appendix 1 to LLNL report UCRL-ID-146938, dated Feb. 1, 2002, “Concluding Report: Quantitative Tomography Simulations and Reconstruction Algorithms”, Wittenau et al report the results of Monte Carlo simulations of image blur due to scattering from the front and back covers of a detector. In LLNL report UCRL-ID-147751, dated Feb. 25, 2002, “Edge-Spread Functions Expected for Several Changes in a Commercial Flat-Panel System”, Wittenau reports the result of Monte Carlo simulations performed to predict the effect on image quality from certain modifications (thinner housing, decreased air gap, etc.). Neither of these reports is based on actual data, and the improvements described therein were never implemented, to the best of our knowledge. Even if they had been implemented, the improvements described therein are not same as the improvements of this invention, nor would they achieve the same quality of results.
In the following, background information is given on (1) the basics of flat panel detectors, the modification of which is the subject of this invention, and (2) the problem that the invention is designed to solve, namely, to measure attenuations accurately and perform beam hardening corrections, and (3) data which show that the cause of the problem is scatter interior to the flat panel detector, (4) how that scatter can be greatly reduced by modifying the flat panel detector, and (5) the benefits of the improved flat panel detector design described herein.
1. The Basics of Flat Panel Detectors
Flat panel x-ray detectors (herein called “FPDs”) are used to create a digital x-ray image of an object to be examined (herein called the “object”) that is placed between an x-ray source and the FPD. There are several existing FPD designs. A common variety, termed an indirect x-ray detector, is based on a 2-D amorphous silicon or CMOS light detector array in contact with a scintillator. The scintillator converts a spatially varying x-ray intensity behind the object to a spatially varying light intensity, which results in an image of the object. The light image is detected by a array of photodiodes imprinted on the amorphous silicon or the CMOS array. The photodiodes convert the light to an array of electrical signals that are processed to form a digital image. Other FPD designs may utilize direct conversion of x-rays to electrical signals, for example, through amorphous selenium arrays.
Regardless of the method of generating the digital x-ray images, the digital x-ray detector components are typically assembled with other components that may include a housing, protective covers, supports for the detector glass, electronic circuit boards, cooling components, and shielding. Existing FPDs usually have some of these components located in front of or behind the detector array, in the path of the x-rays that penetrate the object. For example, many FPD designs place the electronic components and shielding behind the detector components to make the assembly more compact. Many FPD designs use sheets of aluminum, plastics, and/or carbon fiber composites, placed close to and parallel to the detector array, for various purposes. These purposes include structural support of the detector glass, dust covers, electrical shielding, and structural function of the housing. The components situated in the path of the x-ray beam cause both qualitative and quantitative imperfections in the digital x-ray image. These imperfections may or may not be important, depending on the application of the image, as discussed below.
In addition to digital radiography (DR), that is, capturing individual digital x-ray images for the examination of an object, flat panel detectors are commonly used to acquire data sets used for x-ray computed tomography (CT). In CT, a multitude of DR images are acquired at different angles of view. This can be done by rotating an object that is placed between a stationary source and detector (a typical industrial application) or rotating the source and detector around a stationary object (a typical medical application). More commonly in the medical application, a helical scan is performed by translating the object (the patient) along the axis of a circle, on which the source and detector are rotated. Regardless of the configuration used to acquire the CT data, the multitude of x-ray images are converted to attenuation images and then processed by a computer, using a procedure known as a reconstruction algorithm, to create a 3-D image of the attenuation coefficient inside the object. Since, with a given x-ray beam energy, the attenuation coefficient is related to the density of a material and its atomic number, the reconstruction gives an indication of the density or material variation in the object. An advantage of CT over DR is that the components of the object, such as bones and flesh, may be seen separately, either in cross sections or as 3-D images, rather than overlapping in a 2-D projection image. Another advantage of CT is that the contrast between components of the object is enhanced.
2. The Problem Solved by this Invention
Reconstructed 3-D CT images are more sensitive than the original DR images to imperfections in the numerical values measured by the flat panel detector. Such imperfections may appear as minor variations of image intensity in a DR image, but these variations are magnified, sometimes to cause significant artifacts or distortions, in the reconstructed CT image.
An example of an artifact that is important to CT but not so important to DR is an artifact caused by “beam hardening.” Beam hardening is, in essence, the change of spectrum of a polychromatic x-ray beam as the beam penetrates the object. Usually the low energy components of the x-ray spectrum are preferentially absorbed near the surface of the object, so the beam becomes higher in average energy, hence “harder” as it penetrates the object.
In the absence of beam hardening, total attenuation through an object consisting of a single solid material should theoretically be a linear function of thickness, as shown by the line Ath in FIG. 1. Beam hardening causes the experimentally measured attenuation to be a nonlinear function of thickness, as shown by the curved line Aexp in FIG. 1.
Most CT reconstruction algorithms do not directly take beam hardening into account. As a result, in a CT reconstruction that does not correct for beam hardening, an object of uniform density will appear to be more absorbing to x-rays, hence more dense, on its outer surface than in the interior. This is the well-known “cupping artifact,” examples of which are shown below in FIG. 2 and FIG. 3 using simulated and real data, respectively. In both cases the CT object was a circular cylinder, and the graphs are plots of attenuation coefficient measured across the diameter of the cylinder. Other manifestations of beam hardening include streak artifacts caused by shadowing.
There are a variety of methods to correct for beam hardening artifacts in CT reconstructions. The most common method is to calibrate the nonlinear behavior of attenuation vs. thickness (the Aexp curve in FIG. 1) using a wedge of the material in question, and to use that calibration curve to modify the measured attenuation images of the object before reconstruction. An example of such a correction is shown in FIG. 2 and FIG. 3 using simulated and real data, respectively, for the same cylindrical objects.
The data used to generate FIG. 2 were calculated using simulations of the x-ray attenuation for a polychromatic beam. The simulation used a known source spectrum, NIST data for the attenuation coefficient of iron for each energy in the spectrum, and the assumption that the x-rays travel in straight lines through the object. The contributions from all energy components are summed to obtain the polychromatic response. These data are perfect and consistent, in that the measurements follow the same theory for both the cylindrical object and the calibration wedge. This makes the wedge correction theoretically exact, as is seen in FIG. 2.
In order for the beam hardening correction to succeed with actual x-ray data rather than simulated x-ray data, it is necessary that the measured attenuations of both the object and of the calibration wedge be accurate and consistent. For this to be true, the measured x-ray data must satisfy the assumption of straight line propagation, which implies that there can be no scattering. Scattering in the object and detector introduces alternative x-ray paths that deviate from the straight line path. As will be shown below, scatter can cause the detector to be illuminated behind an object even though X-rays do not penetrate the object. Scattering distorts the attenuation measurements and causes the beam hardening correction to be inaccurate. FIG. 3 shows an example of a beam hardening correction based on real data obtained from a typical flat panel detector. Even after performing a careful beam hardening correction, there is a significant residual cupping artifact. This underestimation of beam hardening correction is typical of results obtained with flat panel detectors. The errors make it impossible to use existing flat panel CT data for quantitative CT evaluations, such as the measurement of a density profile. Other beam hardening correction methods exist. These might include, for example, a theoretical accounting of attenuation from various physical attenuation mechanisms, considering the source spectrum and the detector response spectrum in detail. U.S. Pat. No. 6,256,367 discloses a method using Monte Carlo simulation to derive corrections for X-ray scatter from the object. This does not address the scatter from detector components, which is the function of this invention. U.S. Pat. No. 7,065,234 discloses a method for beam hardening correction through use of a simulated attenuation curve that includes a known detector response function. Regardless of the method used for beam hardening correction, the method must rely on the assumption that the shape of the attenuation image measured by the detector in the CT scan is reasonably accurate. Errors in the shape of the attenuation image caused by scatter in the detector will result in inaccurate beam hardening correction.
Physical means have been used to correct for scatter. U.S. Pat. No. 6,618,466 discloses use of a bow-tie filter to reduce beam hardening artifacts. The bow tie filter is intended mainly to reduce dose to the patient. It reduces scatter from the object, but it does not address the issue of scatter caused by the components of the detector. U.S. Pat. No. 6,744,852 discloses use of an anti-scatter grid comprised of a high-resistance foam. Anti-scatter grids are intended to solve the problem of scatter from the object, and they do not address scatter caused by components of the detector. Published U.S. Patent Application 2007/0085015 discloses a detector housing comprised of graphite fiber-epoxy composite. Although a housing of solid sheet carbon fiber composite (CFC) could have lower scatter than an aluminum housing, our experimental data show that significant scatter would still result because of the thickness and density of the solid sheets of CFC. Application 2007/0085015 also suggests the inclusion of X-ray absorbing material behind the detector plane. This would be a significant source of backscatter, and that suggestion indicates a lack of appreciation of the need for transparency throughout the detector. There is still a need for a truly transparent housing on the both front and the back of the detector and reduced x-ray absorption from all components internal to the detector in order to solve the detector scatter problem.
3. Data which Show the Cause of the Problem
The fact that scatter adversely affects the attenuation measurements and that the flat panel detector can be the primary and most significant source of the scatter is demonstrated in the following section.
FIG. 4 shows two different attenuation measurements made on the same copper wedge object, under identical x-ray exposure conditions, using the same flat panel detector for the measurements. The wedge measures 4 in. long×2 in. wide, with a thickness at the base of 2 in. (50.8 mm). The exposure was at 420 kV, with a 2.18 mm copper pre-filter. The only difference between the two measurements is the extent of the area of the FPD that was exposed to x-rays. One measurement was done with the x-ray beam shaped to the full size of the active detector area of the FPD. The background exposure of the FPD produced a significant scatter signal behind the wedge. The second measurement was made using a highly collimated pencil beam, for which there is very small scatter in the FPD, and scanning the object through the pencil beam.
At a wedge thickness of 25 mm, the two attenuation curves in FIG. 4 have a ratio of around 4.3/1.8=2.39. This means that the transmitted x-ray signal behind the wedge have a ratio of exp(2.39)=11 between the two curves. An 11/1 signal ratio means that 91% of the signal behind the uncollimated wedge was scatter, and only 9% was direct transmission through the wedge. Obviously, scatter has a large effect on attenuation measurements for highly attenuating objects.
Another demonstration of the scatter is seen in a radiograph of a two-inch thick lead brick, through which there should be negligible transmission signal at 450 kV. FIG. 5 shows the measured edge profile of X-ray signal across the edge of the brick. FIG. 5 shows that the scatter signal behind the brick is around 400 out of 4095, or 10% of the unattenuated signal near the edge, and decays over a long distance. The false signal near the edge corresponds to a false attenuation measurement of around ln(4095/400)=2.32. About 1 inch in from the edge, the false attenuation is around ln(4095/200)=3.01. In the absence of scattering, the signal behind the lead brick should be near zero, and the attenuations should be very large. These large errors in attenuation, caused by scatter, explain the large difference between the two attenuation curves for the copper wedge in FIG. 4.
Further insight into the cause of the problem is gained by observing that the scatter curve in FIG. 5 is nearly up-down symmetric. This means that the scatter is not forward scatter from the object. If it were, there would be a tail outside the lead brick but zero signal behind the lead brick. The only way for photons to scatter to locations behind the lead brick is for the scatter source to be located behind the lead brick.
The reason for the symmetrical tails is illustrated in FIG. 6. In the absence of the lead brick, the detector plane is bathed in a uniform field of direct x-ray impingement and scatter. FIG. 6 shows the scatter to be backscatter, coming from an aluminum plate behind the detector plane that is representative of an aluminum housing. This is a schematic of one of the MCNP photon transport models that we developed to study this effect. [MCNP is a Monte Carlo (probabilistic) particle transport code developed at Los Alamos National Laboratory. Particles that may be tracked as they travel through an object include, among others, neutrons and x-ray photons. Particle interactions with matter are modeled using probabilistic particle cross section libraries. MCNP can be used both for ray tracing and scatter calculations.] The symmetric scatter tail would also occur with forward scatter from FPD components in front of the detector plane. Regardless of the direction of scatter, the insertion of the lead brick into the x-ray field shadows the direct x-ray field on the detector, hence the source of the scatter in the detector. Scatter coming from the fully exposed part of the FPD shines on the detector behind the brick. Lack of a direct scatter source behind the brick reduces the signal outside the brick in the same way. The effect is symmetrical. At the edge of the brick the scatter signal is half the value it is for the fully exposed FPD without the brick. If the magnitude of the tail is 10%, the level of scatter in the fully exposed FPD is 20%. Thus, a truly significant fraction of this FPD's reading in the unattenuated background region, and also behind objects, is scatter.
4. Data that Demonstrate the Method to Fix the Problem
We have performed several studies with simulation and laboratory experiments which demonstrate that most of the scatter in our CT images is coming from components of the FPD itself. In particular,                We have used the MCNP photon transport code to show that the scatter from the room, especially the back wall, is orders of magnitude lower than the scatter from the aluminum housing of the detector.        We removed the aluminum housing from the front and back of the detector plane in an old FPD and found that the scatter tails shown in FIG. 5 were reduced by a factor of two or three, depending on x-ray energy.        We have selectively added and removed materials, such as solid, dense CFC panels of the type normally used on FPDs, from the front and back of the detector glass, demonstrating that scatter signal from these materials is both significant and occurs about equally in the forward and backward direction.        We have inserted light weight CFC-foam sandwich structures in the beam path and found that their effect on the scatter tails is negligible, validating this choice of a transparent material.        Some existing detectors, including the one we used for our experiments, have an aluminum plate that supports the detector glass. We believe that this last aluminum component, which we could not remove, is responsible for most of the remaining scatter that we observed after removing the housing. This is based on our knowledge that the scatter occurs nearly isotropically over a wide angle and is not significantly reduced by the proximity of the glass support to the glass.        
Alternative solutions exist to the FPD scattering problem. A linear array of collimated point detectors gives excellent, low-scatter results. This solution is rejected for our applications because data acquisition with a linear array is very much slower and less convenient than with a flat panel area detector. The purpose of this invention is to take advantage of speed and convenience a flat panel area detector yet improve its design so that its images will be of higher quality.
5. The Benefits of the Improved FPD Design
The benefits of the improved FPD design are as follows:                The long distance blurring of the DR images will be eliminated. Fine details will be seen more easily.        The quantitative attenuation values of the images will be much closer to correct, so the images can be used for quantitative DR and CT studies.        It will be possible to make more accurate corrections for beam hardening, especially with highly attenuating objects. This will improve image quality of CT reconstructions.        The dynamic range of the detector will be increased by reducing the background error. Satisfactory x-ray images could be obtained with lower exposures, or alternatively the signal to noise ratio could be increased.        In view of the above, patients may receive lower dose in diagnostic x-ray imaging.        